Security instruments such as identification cards and sheets normally include one or more printed images which are used to identify the cardholder and authenticate the security instrument. For example, a security instrument in the form of an identification card, a driver's license or personal identification sheet of a passport may be made of a thermoplastic substrate and bear a printed photograph of the person to whom the card, license or passport was issued to identify and verify a person holding the instrument. Typically, this photograph may be printed onto the thermoplastic substrate by means of ink jet printing using an ultraviolet (UV) curable ink. According to presently used ink jet printing methods, the UV cured ink jet image is applied to the surface of the thermoplastic substrate of the card which has a generally smooth or matte surface. After the personalization image has been printed onto the surface of the substrate a clear coating, or patch, may be printed over it to provide additional protection to the image. A protective patch typically comprises a UV cured varnish which is also applied by means of ink jet printing. Alternatively, hot lamination of a polyester patch is sometimes used. Disadvantageously, however, using a patch will increase the manufacturing cost for the security instrument and, depending upon the application, the patch may be prone to separate from the image.
To improve the adherence of the printed image to the surface of the thermoplastic substrate of a card an adhesion promoter may be used. It has been found, however, that the resulting images provide poorer characteristics of adhesion than desired.
One durability test method that is used for secure instruments comprising printed images, as an indicator of the wear resistance of the image is the Taber Abrasion Test. For example, using this test, a thermoplastic identification card comprising a personalization image printed by means of ink jet printing is typically expected to withstand 5,000 abrasion cycles during testing, without any degradation of the personalized areas on the card including those areas on which the personalization image appears.
In addition to improving both the adhesion and durability, it is desirable that personalization images (i.e. images used to authenticate a person's identity) applied to secure instruments be resistant to tampering by forgers, such as by replacing an authentic, personalized image with a non-authentic image. Increased security may be provided by incorporating predetermined features for detection and verification of the instrument for authenticity which are difficult to reproduce by a forger.
Therefore, it is desirable to provide one or more of the characteristics of improved adhesion, durability and security to secure instruments having surface applied printed images thereon.